三好生 is a honor for chinese students.. 三好学生 should be good at study, virtue and sports... this sentence is a congratulation for someone who gets the honor.

祝贺 你 当上了 三好生！ = Congratulations on being a successful student!

the 三好生 concerns a typical rating system to evaluate the performance of students in elementary school. It is rare that we praise someone in middle school or with higher education level as a 三好生. So please keep in mind that 三好生 literally refers to the student who ranks A-level, respects the dos and donts and keeps a good health, especially in elementary school.

Congratulations! 三好学生= 学习好，思想好，身体好

Congratulations for your being an excellent student!